High Temperature Oxidation and Chlorination of FeCrAl alloys
Doctoral thesis, 2014
The constant strive for improved efficiency and lower-cost industrial processes often results in progressively higher temperatures and more aggressive environments. High- temperature corrosion is a well-known problem in biomass- and waste-fired boilers and is one of the obstacles to overcome in achieving a more sustainable society. Alkali- and chlorine- induced corrosion of chromia-forming alloys has been studied by several researchers. However, the literature is scarce on how alumina forming-alloys, e.g. FeCrAl alloys, perform in such an aggressive environment. In this thesis, the KCl-induced corrosion of FeCrAl alloys is studied through well-planned laboratory exposures, detailed analyses and corrosion tests in a full scale waste-fired power boiler.
In the laboratory, the effect of adding small amounts of KCl was investigated on polished FeCrAl samples in O2 or O2 + H2O environments up to 168 hours at 600 °C. Pre-formed alumina scales were prepared on the alloy substrate through pre-oxidation at various conditions. The pre-treated samples were subsequently exposed to O2 + H2O + KCl in order to investigate their corrosion resistance. The samples were analysed using TGA, SEM/EDX, XRD, IC, AES, SIMS, BIB cross sections and STEM/EDX. In order to evaluate the validity of the findings in the laboratory studies, corrosion tests were carried out in a full-scale combined heat and power (CHP) boiler.
The study showed that KCl-induced corrosion caused a rapidly growing iron-chromium-rich oxide to form in O2 and O2 + H2O environments at 600 °C. Chromate formation and alloy chlorination were found to initiate the formation of a non-protective oxide scale. Alloy chlorination was greater in O2 than in O2 + H2O resulting in a more porous scale with poor adhesion to the alloy substrate.
Pre-treating the alloy prior to exposure to O2 + H2O + KCl mitigated the corrosion, since alumina, itself, is rather inert towards corrosion. However, the corrosion started locally, most likely at flaws/cracks in the alumina scale and then spread laterally until the entire surface suffered from breakaway corrosion. Similar results were obtained in the corrosion tests in the waste-fired boiler. The pre-formed alumina scale failed after only 24 hours in the boiler at both 600 and 700 °C.